The present invention relates generally to magnetic field generators for magnetic resonance imaging (MRI) devices and systems, and, more particularly, to a system and method of assembling a single polepiece from a plurality of magnetic tiles such that the tiles of the polepiece are restricted from disassembly.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
To generate these high uniform magnetic fields, many MRI systems utilize a permanent magnet system capable of generating a uniform magnetic field on the order of 0.2 to 0.5 Tesla and higher within a pre-determined space or imaging volume. Generating the desired magnetic field during an MRI process induces electric eddy currents on the permanent magnet system. These electric eddy currents can create distortion in the imaging data that may serve to severely degrade the quality of a reconstructed image. To limit the induction of eddy currents during MRI imaging, the permanent magnet system may be constructed of multiple blocks or tiles that are, in turn, constructed of thin, stacked, sheets or laminates. The laminates are typically bonded together to form a single laminate structure.
Since the tiles are typically fabricated or otherwise formed of a ferromagnetic magnetic material and the tiles are exposed to strong magnetic fields during imaging, the large magnetic forces generated may act upon tiles in an undesirable manner. That is, over time the magnetic forces may cause the tiles to pull apart or delaminate. To counter the impact of these magnetic forces, the tiles are generally bonded together. Ideally, the bonding strength between tiles would be sufficient to counter the delaminating forces imposed by the strong magnetic fields. To sufficiently bond the layers, however, requires that each and every tile and every layer of each tile be sufficiently bonded. To ensure that the adjacent tiles and that the layers of each tile are sufficiently bonded can be an arduous and cost-prohibitive process.
Therefore, it would be desirable to have a system and method of sufficiently securing the tiles to one another in a manner to counter the delaminating or otherwise disassembly forces that act upon the tiles during magnetic field generation without substantial increases in production cost and time.